I was only on Depo for 9 mo's and I hated it! I also gained about 30 lbs incredibly quickly, same stretch mark thing too. Pissed me off! So anyway, my Depo didn't wear off until almost a year. Then I just suddenly dropped like 10-15 lbs. But I've never gotten back to the 120lbs I was before Depo. You may be more lucky than I, however.
The IUD sounds interesting, but my PP told me they wouldn't do it for women who haven't had a child because it was too painful and more likely to come out. You haven't had kids though, have you?